The invention relates to a process for aerating a liquid with gas, particularly for use in processes for separating impurities from suspensions by means of flotation, covering the injection of liquid through a nozzle into a volume of gas, as well as a device for aerating a liquid with gas, particularly for use in plants or equipment for separating impurities from a suspension by flotation, covering a pressure vessel for the liquid and a gas volume above it, with a pressure connection for the gas in the upper section of the pressure vessel and a nozzle through which to inject the liquid into it.
Flotation is a physical-mechanical process for removing impurities from suspensions. This process requires gas bubbles to be generated in suitable quantities and size distributions. Hydrophobic or waterproofed substances are carried to the surface of the liquid by the gas bubbles adhering to them and can be removed there together with the froth. Processes of this kind are known, for example, from DE 41 16 916 C2, and have reached a high technical standard. Self-priming injectors are often used to generate the gas bubbles and mix them into the suspension. Here, the flow of suspension charged with impurities exiting from a nozzle generates a vacuum and sucks in gas, which is mixed with the suspension as a result of the pulse exchange between gas and liquid. A device of this type is described, for example, in DE 34 12 431 A1.
A particular problem in the flotation process is how to make available the liquid saturated with gas and in which the gas bubbles are then to be generated. In EP 789 672 A1, a device is disclosed in which pressurised liquid is sprayed into the top section of a pressure vessel through a nozzle, while this top section is full of pressurised gas. Here, the liquid is only in contact with the gas for a short period so very little gas is introduced to the liquid and too little turbulence is generated. As a result, the liquid must typically be circulated several times or higher saturation pressures are needed, which leads to higher energy consumption and higher energy input being required in the subsequent flotation stage. In addition, gas bubbles may also be discharged in the outflow of saturated liquid, which drastically reduces the efficiency of subsequent flotation.
Further from AT 407 844 a process for aerating dispersions is known with flotation cells with a free surface of the suspension where the froth is separated from the surface with an overflow. Also DE 198 45 536 shows a process where gas is mixed with the liquid. As gas advantageously air, which is to be dissolved in the liquid, is used. Due to the existing high flow velocity only a little portion of the available air can be dissolved in fact. Further the air is not sucked in from a volume under pressure, so that in addition the solubility of the gas/air is only little. The DE 40 29 982 shows a couple of variations of nozzles where special devices, as perforated tubes are used for better distribution ob the air in the suspension. By such a construction an additional pressure drop on the gas side is created leading to a highly reduced solubility in the liquid. The suction of the gas/air by the liquid is also strongly affected by this special distribution device. If the liquid is a fiber suspension blocking of the bores may arise in addition.